Russia's main security agency says it foiled a terrorist attack in Moscow by a group of Russian citizens, several of whom it claims were trained in Syria by the Islamic State group.

The Federal Security Service (FSB) said Monday that it raided an apartment in the Russian capital where six to 11 people had periodically lived, after receiving information the group was plotting a terrorist act targeting Moscow's public transportation system.

The FSB said that during the raid it detained a group of men, some of whom had gone through "combat training" in Islamic State camps in Syria and returned to Russia long before it launched its military operation in the Middle Eastern country late last month.

The security agency said it also seized 5 kilograms of an ammonium nitrate-based explosive and an electronic detonator, among other bomb components.

Asked whether President Vladimir Putin had been informed about the incident, Kremlin spokesman Dmitry Peskov said Russia's special services constantly inform the president about anti-terrorist measures and operations.

Russia began a campaign of airstrikes in Syria on September 30. While it says it is targeting the Islamic State and other terrorist groups, U.S. officials say 90 percent of the Russian airstrikes have targeted opposition groups seeking the ouster of Syrian President Bashar al-Assad, not the Islamic State or al-Qaida affiliated groups.